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D. C. Hood, M. A. Lazow, C. E. Lin, K. G. Locke, X. Zhang, D. Birch; Retinal Nerve Fiber Layer Thickness in Patients With Retinitis Pigmentosa. Invest. Ophthalmol. Vis. Sci. 2009;50(13):983.
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To better understand the effects of retinitis pigmentosa (RP) on the retinal nerve fiber layer (RNFL), patients with a range of visual field defects were studied with frequency domain optical coherence tomography (fdOCT). The integrity of the RNFL is vital for prosthetically-aided vision and reported RNFL findings differ.[1-3]
FdOCT (Spectralis, Heidelberg) horizontal line scans of the midline and circular scans of the optic nerve head (ONH) were obtained from 31 patients with RP and 20 control subjects. Patients’ eyes fell into two groups: 15 had moderate visual field loss (M) (foveal sensitivity ≥32 dB; visual field diameter ≥10°) and 16 had more severe field loss (S) (foveal sensitivity ≤25 dB; visual field diameter ≤ 3°). The median visual acuities were 20/25 (M) and 20/200 (S). Raw images were exported and the thickness of the RNFL measured with a manual segmentation procedure aided by a computer program.
For the ONH scans, the means of the average RNFL thicknesses of groups M (123.8 ± 17.1 mm) and S (125.8 ± 24.1) were significantly greater (p<0.001) than the mean of the controls (102.3 ± 12.3), but not significantly different from each other. Five of the M eyes and 9 of the S eyes had a RNFL thickness greater than any of the 20 control eyes; only one was thinner than any normal. For the horizontal line scans, the means of the average RNFL thicknesses (nasal retina 0 to 3mm) of groups M (31.6 ± 6.0 mm) and S (43.7 ± 11.7) were significantly greater (p<0.006) than the mean of the controls (26.9 ± 3.3), and the S group’s RNFL was significantly thicker than that of the M group (p<0.002). Over 80% of the S group and 40% of the M group had an average RNFL thickness greater than any of the 20 control eyes; only one was thinner than the control values.
Individual patients have a RNFL thickness that is equal to, or greater than, normal. Further, average RNFL thickness is greater than normal in patients with RP, and greater along the horizontal meridian in patients with severe visual field loss, as compared to those with moderate field loss. Before accepting RNFL measures as predictive of utility in prosthetic procedures, we need to determine whether thickening is due to neural remodeling or to mechanical factors such as stretching, neuronal swelling and/or edema. 1. Walia et al, IOVS, 2007; 2. Walia, Fishman IOVS, 2008; 3. Hood et al, IOVS, E-pub
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